The spark plug is attached to a combustion apparatus in the internal combustion engine or the like and used to ignite an air/fuel mixture within a combustion chamber. Generally, the spark plug includes an insulating body that has an axial hole, a center electrode that passes through the axial hole, a metal shell that is arranged at the outer periphery of the insulating body and a ground electrode that is arranged at a leading end surface of the metal shell and forms a spark discharge gap with the center electrode. Also, when the metal shell and the insulating body are assembled, generally a step section that is arranged at the inner periphery surface of the metal shell and a tapered section that is arranged at the outer periphery surface of the insulating body are engaged through a metal sheet packing.
However, carbon is generated by an incomplete combustion of the air/fuel mixture in the combustion chamber and there is concern that the carbon will be deposited on the surface of the insulating body. When the carbon deposit advances to the surface of the insulating body, the leading end section surface of the insulating body is covered by carbon and then contaminated, and there is concern that a normal spark discharge may not be generated in the spark discharge gap and electric current may flow (leak) via carbon to the metal shell from the center electrode.
Meanwhile, a spark plug, which has a function of burning and eliminating the carbon, in other words a “self-cleaning function”, is known in which a surface temperature of the insulating body is raised rapidly in order to enhance a contamination-resisting property.
However, when the temperature of the leading end of the spark plug is excessively heated above a predetermined temperature (for example, 1100° C.), there is concern that the overheated leading end of the spark plug becomes a spark source. In other words, the air/fuel mixture is ignited, in other words, “pre-ignition” is randomly generated even before ignition of the spark plug.
Accordingly, a technique has been proposed in which the gap between the portion that is positioned at the leading end side of the tapered section and the metal shell is small and a length along an axis of the gap is large (for example, see Patent document 1). In the technique, the gap is small so that the heat is controlled, so as to be effectively transferred toward the metal shell from the insulating body and enhancement of heat-resisting property is promoted. Also, the length along the axis of the gap is large so that inflow of unburned gas (carbon) toward the gap is prevented and enhancement of the corruption-resisting property is promoted.